The present invention relates to optical barcode readers and includes an optical signal processing detector that results in a cleaner, more linear electrical signal at the photodiode level.
Barcode reading systems, as known in the art, employ a finely focused light beam scanned across a coded label located in the field of view of a photodetector. The label represents information encoded as a series of bars of various widths formed on a contrasting background. The difference in reflectance of the bars versus the spaces produces a modulated optical signal. The optical signal is converted to an electrical signal by a photodetector. This electrical signal is further processed and digitized before decoding.
As barcode scanner technology has advanced, the quality of label necessary for accurate reading has decreased. This is highly advantageous in that labels can now be printed by conventional printing methods directly on packaging. Alternatively, computer printed labels can be employed to further automation.
Regardless of the system design, all barcode reading systems must address several common, inherent problems:
A major concern lies in the nature of the light reflected from barcode labels. Most of the light returned to the detector is diffuse, due to the optically rough surface of the label. It is the difference in this diffuse light to which the detector is sensitized. When labels are printed on smooth, shiny surfaces or are oriented in a particular way, mirror-like reflections can be returned to the detector. These intense specular reflections can overpower the diffuse light signal and introduce a major source of error.
A second problem is ambient light. Strong interior lighting or bright sunlight can also overpower the detector and further decrease the signal to noise ratio.
A further design consideration is the ideal that the detector receive the same intensity of diffuse light from all points along the scan width. This is a difficult objective to achieve because considerably more light is returned from the central area of the scan than from the periphery.
Shotnoise is an additional consideration. Technically, shotnoise is characterized by fluctuations in the current leaving the photodiode. Shotnoise increases with increased photodiode area. Furthermore, when the received light signal is less concentrated over a large photodiode area the signal to noise ratio also decreases.
In order to realize the benefits of inexpensive labels and to compensate for these adverse optical signal conditions, barcode reader designers have evolve increasingly expensive and complicated electronic signal conditioning techniques. This invention shows that these shortcomings can be addressed more economically by optical means, resulting in a more accurate electrical signal at the outset.
Some of the problems discussed above have been addressed, to a limited extent, in the prior art.
U.S. Pat. No. 2,018,963, Land, disclosed the use of a polarization technique to attenuate specular reflections. The method involves polarizing the light source normal to the plane of incidence and viewing the image with an analyzer crossed with the polarizer. U.S. Pat. Nos. 3,812,374, Tuhro, 3,801,182, Jones, and 3,502,888, Stites, employ this technique in various forms. All are based on the fact that specular reflections maintain the same polarization as the incident beam whereas the diffuse light is randomly polarized and will not be absorbed by an analyzer crossed with the incident polarizer. The present invention forgoes the need for an initial polarizer because the diode laser output is inherently polarized.
U.S. Pat. No. 3,746,868, Plockl, disclosed the use of narrow-pass wavelength filters as a means to improve the signal to noise ratio in an optical reader by filtering out light of a different wavelength than the optical reader light source, e.g. ambient light. This effective, inexpensive technique is further improved in the present invention.
In light of these considerations, it is an object of the present invention to provide an optical barcode reader operable in a wide range of ambient light conditions.
It is a second object of the invention to provide an optical barcode reader desensitized to specular reflections.
It is a further object of the invention to provide an optical detector assembly for a barcode scanner that gathers a uniform intensity of diffuse light throughout the scan angle.
It is an additional object of the invention to provide an optical detector assembly for a barcode reader that minimizes shotnoise produced by the photodiodes.
It is yet another object of the invention to provide an effective optical solution to the inherent adverse optical signal conditions encountered by barcode readers, thereby eliminating the need for expensive and complicated electronic signal conditioning techniques.
Other objects and advantages of this invention will become apparent when the following detailed description is taken in conjunction with the drawings and appended claims.